This invention relates to a method and a composition for inhibiting the heat degradation of polypropylene polymers filled with asbestos reinforcing fibers. More particularly, the invention relates to stabilized filler formulations, a method of incorporating asbestos fiber and stabilizer during plastication of polypropylene, and to polypropylene products possessing desirable physical characteristics.
Asbestos fibers possess many desirable properties for use as a reinforcing filler for polypropylene. They improve the hardness, stiffness, and heat deflection of polypropylene compositions in which they are incorporated. However, asbestos fibers when used as reinforcing fillers for polypropylene possess one major disadvantage--the promotion of polymer instability at elevated temperatures.
Polypropylene is commonly exposed to elevated temperatures in making useful compositions and articles out of the resin form, and also in normal uses of some of these compositions and articles. Such customary processing procedures as roll compounding, injection molding, extrusion and the like involve elevated temperatures. In such end uses as electrical insulation, protective coatings for electrical wire, and plastic pipes for hot water and steam, elevated temperatures are frequently and normally encountered.
Despite the fact that asbestos fibers tend to accelerate polymer degradation at elevated tempertures, asbestos fibers are widely used in polypropylene as a reinforcing filler. The polypropylene heat degradation problem has been ameliorated in the past by utilizing as fillers varieties of amphibole asbestos such as anthophyllite, in conjunction with chemical stabilizer systems.
While the cost of chrysotile asbestos is only 40 to 50 percent of the cost of anthophyllite, chrysotile's greater exposed surface area and potential for chemical reactivity have generally required economically prohibitive amounts of chemical stabilizers to achieve heat-degradation properties that even approach the properties of anthophyllite-filled polypropylene. In other words, to achieve the same heat stability properties, the cost of the stabilizers needed for use with chrysotile have generally exceeded the sum of the cost of the stabilizers needed for use with anthophyllite and the cost differential between the anthophyllite and chrysotile. Thus, there has existed a need for improved stabilizer compositions, and for methods of inhibiting the heat degradation of asbestos-filled polypropylene.